1. Field of the Invention
The present invention relates a longitudinally coupled resonator-type surface acoustic wave device and more particularly, to a longitudinally coupled resonator-type surface acoustic wave device in which the configuration of electrode fingers of adjacent IDTs (interdigital transducers) is improved.
2. Description of the Related Art
Recently, resonator-type surface acoustic wave devices in which unnecessary modes causing ripples are suppressed have been demanded.
Conventionally, in longitudinally coupled resonator-type surface acoustic wave devices, since a transverse mode causes ripple, a method for applying weighting to IDTs, for example, in accordance with finger overlaps (the amount of overlap of the electrode fingers along the length of electrode finger of IDT), i.e. apodization weighting, has been used to suppress transverse modes. However, when a transverse mode is suppressed by applying weighting to IDTs, the effect on the principal propagation mode is large, and accordingly it is difficult to obtain the desired filtering characteristics.
On the other hand, in a surface acoustic wave resonator described in Japanese Examined Patent Application Publication No. 6-85492, the finger overlap of electrode fingers of the IDTs is set to be 65% to 75% of the electrode finger length in order to suppress ripples caused by a transverse mode. Furthermore, in a resonator-type surface acoustic wave filter described in Japanese Unexamined Patent Application Publication No. 9-260996, the finger overlap of the electrode fingers of the IDTs set to 75% to 85% of the overall electrode finger length to suppress ripples caused by a transverse mode.
As described in Japanese Examined Patent Application Publication No. 6-85492 and Japanese Unexamined Patent Application Publication No. 9-260996, when the ratio of the finger overlap of the electrode fingers to the overall electrode finger length is reduced, the area where a surface acoustic wave is excited is reduced which adversely effects the propagation mode.
To overcome the problems described above, preferred embodiments of the present invention provide a longitudinally coupled resonator-type surface acoustic wave device in which the effect on the principal propagation mode and transverse modes are effectively suppressed.
According to a preferred embodiment of the present invention, a longitudinally coupled resonator-type surface acoustic wave device includes a surface acoustic wave substrate, and a plurality of IDTs, each having a plurality of electrode fingers, which are arranged in the propagation direction of a surface acoustic wave on the surface acoustic wave substrate. In the longitudinally coupled resonator-type surface acoustic wave device, an electrically discontinuous portion is provided in a central portion, in the finger overlap direction (the vertical direction in FIG. 1), in adjacent electrode fingers of adjacent IDTs and at least one floating electrode is provided in the finger overlap direction in the electrically discontinuous portion.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, the ratio b/a is preferably at least about 0.575, where the length in the finger overlap direction of the electrically discontinuous portion is designated by a and the length in the finger overlap direction of the floating electrode is designated by b. Then, not only a ripple caused by a transverse mode is effectively suppressed, but also the reduction of insertion loss on the high frequency side in the passband is prevented.
According to a second preferred embodiment of the present invention, a longitudinally coupled resonator-type surface acoustic wave device includes a surface acoustic wave substrate, and a plurality of IDTs which are arranged in the propagation direction of a surface acoustic wave on the surface acoustic wave substrate. In the longitudinally coupled resonator-type surface acoustic wave device, in one electrode finger of adjacent IDTs, an electrically discontinuous portion is provided in a central portion in the finger overlap direction.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, each of the electrode finger portions on both sides in the finger overlap direction of the electrically discontinuous portion is connected to adjacent IDTs. Therefore, a portion which is excited, a portion which is not excited, and a portion which is excited are disposed in the finger overlap direction in that order in the electrode fingers where an electrically discontinuous portion is provided and a transverse mode is greatly suppressed.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, the length in the finger overlap direction of the electrically discontinuous portion is at least about 0.6xcex. Accordingly, a transverse mode is more effectively suppressed.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, the length in the finger overlap direction of the electrically discontinuous portion is about 7.5% to about 87.5% of the finger overlap. Accordingly, a transverse mode is more effectively suppressed.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, the surface acoustic wave substrate includes two opposing end surfaces at which a surface acoustic wave is reflected. Therefore, an end surface reflection type surface acoustic wave device in which adverse effects on the principal propagation mode are minimized and a transverse mode is effectively suppressed is obtained.
In a longitudinally coupled resonator-type surface acoustic wave device of preferred embodiments of the present invention, a plurality of longitudinally coupled resonator-type surface acoustic wave devices may be connected in series, however, at least one of the devices must be a longitudinally coupled resonator-type surface acoustic wave device according to preferred embodiments of the present invention so as to suppress a transverse mode.
Furthermore, in another preferred embodiment of the present invention, a communication device includes a longitudinally coupled resonator-type surface acoustic wave device according to the above-described preferred embodiments of the present invention which defines a bandpass filter. Since a longitudinally coupled resonator-type surface acoustic wave device, in which adverse effects on the principal propagation mode are minimized and a transverse mode is effectively suppressed, is provided as a bandpass filter, a communication device having outstanding communication capabilities is provided.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.